1. Field of the Invention
The present invention relates to magnetostrictive transducers or gauges. The magnetostrictive position transducer gauge which is disclosed incorporates several novel subsystems. More particularly the preferred transducer contains, a low power pulse generator module; a high resolution time measurement module; and a position measurement to current conversion module. These structures cooperate together to produce a "4-20 ma transmitter standard", magnetostrictive linear position transducer.
2. Description of the Prior Art
In general, magnetostrictive position sensors include a ferromagnetic delay line, which is occasionally called a "waveguide" A pulse generator supplies a current pulse to the delay line which generates a magnetic field which surrounds the delay line. A remote and movable position indicating magnet is positioned along the delay line. The magnetic field of the position magnet disturbs the magnetic field generated by the current pulse.
The interaction between the permanent magnetic field of the position magnet and the magnetic field induced by the current pulse causes a strain or mechanical reaction within the delay line. This strain induced reaction force within the delay line, is propagated along the length of the delay line as an acoustic torsional wave.
A detector, called a mode converter, is typically attached to one end of the delay line. This element detects the passage of the torsional acoustic wave and converts it into a representative electrical signal.
The time delay period from the excitation of the waveguide to the reception of the corresponding acoustic wave at the mode converter indicates the position or location of the position magnet along the length of the delay line.
A variety of time measurement, or intervalometer techniques have been used to convert the time period information into a position indicating signal.
For example, U.S. Pat. No. 3,898,555 to J. Tellerman, uses a fixed frequency oscillator to excite the delay line. The returned acoustic signal, in conjunction with the fixed frequency oscillator, develops a signal which is "pulse width modulated" by the position of the magnet along the delay line. An integrator converts the pulse width modulated waveform to a dc voltage level which forms the transducer output.
U.S Pat. No. 4,721,902 to J. Tellerman et al. teaches inter alia, a method to convert the "pulse width modulated signal" into a digital value. The patent teaches the use of a conversion counter to collect "counts" from a conversion oscillator during the "on" time of the pulse width modulated signal.
This patent also teaches a method to enhance position resolution by utilizing the returned acoustic signal to initiate the next excitation pulse. In this fashion a "burst" of interrogation pulses are used to accumulate a large number of "counts" to enhance the resolution of the position signal.
Magnetostrictive position sensor devices of this type are used in the measurement and control industry and find use in machine tools, robotics, liquid level indicators as well as other applications.
To facilitate the use of various types of transducers, produced by a variety of manufacturers, industry has adopted a current mode transducer standard referred to as the "4-20 milliamp transmitter" standard. Under this standard, transducers are supplied as a two terminal device. In use, the two terminal transducer device is coupled to a power supply (24 volts, D.C.) and the amount of current drawn by the transducer from the power supply indicates the measured value of the transduced signal. For example, a pressure sensor may draw 4 ma of current from the remote power supply at the minimum pressure, and 20 ma at the maximum pressure, while intermediate pressures would correspond to intermediate current draws.
The magnetostrictive measurement technique requires the delivery of high current pulses to excite the delay line. As a consequence, traditional methods of exciting and operating a magnetostrictive position sensor have been unsuitable for operation in accordance with the 4-20 ma standard.